The invention relates to a device for coupling a light source and an optical waveguide. The light source has an emissive surface whose dimensions are of the same order of magnitude as, or are smaller than the cross-sectional dimensions of the waveguide. The coupling device comprises a ball lens.
The invention is employed for coupling light-emitting diodes and fibers, for example graded index optical fibers. In particular it is used in cases requiring a light-emitting diode mount.
A device for coupling a light-emitting diode and a stepped index optical fiber is described in an article by J. P. Cabanie and J. J. Hunzinger entitled "Les methodes de couplage d'une source et d'une fibre optique" (ACTA ELECTRONICA, Vol, 24, No. 1, 1981/1982, pages 105 to 116). This article describes the use of collimating optical systems in order to optimally fill the aperture of the optical fiber.
One of these systems, given as a preferred embodiment, comprises a completely spherical lens, or ball lens, which is in contact with the emissive surface of the light-emitting diode. This ball lens is said to perform very well because it enables a gain on the order of 5.89 to be attained. On the other hand this lens has several drawbacks.
First of all, the numerical aperture of this ball lens is very large, which gives rise to geometrical aberrations. Further, the diameter of this ball lens is very small to limit insertion losses. As a result of the small diameter, the lens is difficult to handle during optical alignment of the device. Moreover, the three elements (i.e. the fiber, the ball lens and the light-emitting diode) should be in contact with each other, which gives rise to stresses in the material of the light-emitting diode which have an adverse effect on the life of this diode.
Further, it is to be noted that the device is not adapted for use with graded index fibers. The article describes how a graded index fiber is mounted directly in contact with the surface of the light-emitting diode. However, this is not possible if the light-emitting diode has a receding emission surface. Also, it gives rise to the same mechanical problems as the arrangement with a ball lens specified for fibers of the stepped index type.
Neither of these methods proposes means for an easy alignment or means for sealing the light-emitting diode.